The present invention relates to method and apparatus for measuring the characteristics of a sheet-shaped article, and more particularly to method and apparatus for generating the signals having a relationship with the quantity of moisture, which is contained or absorbed in the article, by irradiating the article with a near infrared ray and by detecting the infrared ray having a coaction with that article.
As the sheet-shaped article, for example, paper in a paper machine or a coater is exemplified. One of the indispensable items for administering the quality of the paper is the moisture content measurement in the paper.
FIGS. 1 and 2 are diagrammatical views illustrating the constructions of the known moisture content meters for measuring the moisture contents in the above-specified paper, and FIG. 1 is directed to that of transparent type whereas FIG. 2 is directed to that of multi-scattering type.
In FIG. 1, upper and lower heads 1 and 2 are arranged to face each other such that the upper one 1 constitutes an irradiating unit whereas the lower one 2 constitutes a detecting unit. A sheet of paper 3 is interposed between the upper and lower heads 1 and 2 and is irradiated through an irradiation aperture 4 with an interrupted light of the parallel beam, which is prepared by a lamp 6, a lens 7 and a rotary wheel 8 (or a disc-shaped member formed with a through hole). The light having conducted an interaction with the paper 3 and reached an incident aperture 5 is so condensed by a lens 11 that it is detected by a photosensitive element 12. In the construction thus far described, the optical axis of the optical system of the irradiating unit is substantially aligned with that of the optical axis of the detecting unit. On the other hand, the rotary wheel 8 is formed in substantially symmetrical positions with respect to the axis of rotation thereof with the two through holes, in which a photometric optical filter 10 and a reference optical filter 9 are mounted, respectively. As a result, by the afore-mentioned interrupted light, the paper 3 is alternately irradiated with the light having such a wavelength as is determined by the optical filter 10 (or an infrared ray having a wavelength of about 1.95 microns, which will be called the photometric light) and with the light having such a wavelength as is determined by the optical filter 9 (or an infrared ray having a wavelength of about 1.80 microns, which will be called the reference light). Thus, by the action of the photosensitive element 12, the measurement signals (which are generated to correspond to the photometric light and will be called the M signals) and the reference signals (which are generated to correspond to the reference light and will be called the R signals) are detected in a time-series manner and are fed to an arithmetic unit 13. This arithmetic unit 13 is made to have a function of storing an input and a function of accomplishing a dividing operation thereby to generate M/R signals. These M/R signals are fed to a (not-shown) arithmetic unit of the next stage so that they are subjected to the desired arithmetic operation to generate signals corresponding to the moisture content of the paper. In the construction thus described, the reason why the M/R signals are generated is because the characteristic instabilities of the respective components are to be compensated. By taking the ratio between the M signals and the R signals, more specifically, the influences from the agings and the temperature changes of the lamp 6 or the photosensitive element 12 can be eliminated.
Now, the construction of FIG. 2 will be described in the following.
In the facing arrangement of the upper and lower heads 1 and 2 constituting the irradiating and detecting units, the construction of FIG. 2 is characterized in that the irradiation aperture 4 and the incident aperture 5 are offset from each other and in that the upper and lower heads 1 and 2 have their facing surfaces formed with reflective coating films 16 and 17. As a result, the interrupted light, which is prepared by the lamp 6, the lens 7 and a rotary wheel 8' (which is not equipped with any optical filter, as is different from the construction of FIG. 1), reaches the incident aperture 5 after repeated reflection between the paper 3 and reflective surface 16 and 17. The light thus having reached the incident aperture 5 is divided by an optical divider 18 into two lights, one of which is introduced into an M signal system composed of the optical filter 10, a lens 11' and a photosensitive element 12' but the other of which is introduced into an R signal system composed of the optical filter 9, the lens 11 and the photosensitive element 12. The R signals and the M signals, which are detected by the photosensitive elements 12 and 12', occur simultaneously and are fed, as they are, to an arithmetic unit 13' so that they are subjected to a dividing operation to generate the M/R signals.
Now, the water content measuring operations of paper will be described with the use of the respective system having the aforementioned constructions.
FIG. 3 shows the characteristic curves of GP paper (which is composed mainly of mechanical pulp) and BKP paper (or bleached kraft paper which is composed mainly of chemical pulp). In FIG. 3, curves A are obtained by a transparent type moisture content meter whereas curves B are obtained by a multi-scatter type moisture content meter. On the other hand, FIG. 4 shows the characteristic curves of unbleached paper, newsprint and fully bleached paper, and curves A and B indicate similar recognitions to those of the above. In FIGS. 3 and 4, ordinates indicate the M/R signals (in percentage scale) whereas abscissas indicate the water content MW (in g/m.sup.2 scale).
First referring to FIG. 3, the characteristics of the BKP paper and the GP paper are considerably different, as seen from the curves A, but are little different, as seen from curves B. In view of FIG. 4, on the contrary, the characteristics of the unbleached paper, the newsprint (containing deinked pulp) and the fully bleached paper are substantially identical, as seen from the curves A. As seen from the curves B, however, the characteristics of the unbleached paper and the newsprint are found quite different from those of the fully bleached paper.
According to the moisture content meter of the prior art, therefore, the M/R signals, i.e., the moisture content signals are influenced by the mixture ratio of paper materials or by the optical conditions or paper, e.g., the whiteness, the base weight or the pressure applied. In order to lighten and eliminate those influences, the operator of a paper machine or a coater changes the constant of a moisture content calculating equation in accordance with the kinds (or brands) of paper. However, those changing operations are troublesome, and the afore-mentioned optical conditions may change (without attracting the notice of the operator) even for the same brand of paper. Therefore, it has been difficult for the conventional moisture content meter to continuously measure the moisture content of paper with high accuracy.